Viroporins are a growing class of membrane proteins that are important for viral replication and packaging. These proteins also affect cellular functions, including the cell vesicle system, glycoprotein trafficking and membrane permeability (Gonzalez et al., FEBS Lett., 2003, 552, 28-34). The M2 proton channel is a prototype for this class of proteins that is essential to the survival of the virus (Lamb et al., Wimmer E, editor, Receptor-Mediated Virus Entry into Cells, Cold Spring Harbor, N.Y., Cold Spring Harbor Press, 1994, p. 303-321).
Viroporins are essential components of a variety of viruses including Ebola, Marburg, Bluetongue, African horse sickness, foot and mouth disease, and Japanese encephalitis viruses. In particular, Ebola and Marburg viruses pose a particularly serious threat to human health and are classified as category A biowarfare agents by the Center for Disease Control (CDC) (Khan et al., MMWR, 2000, 49, RR-4, 1-14. VP24 from Ebola and Marburg viruses is an integral membrane protein that possesses viroporin activity similar to the M2 protein (Han et al., J. Virology, 2003, 77(3), 793-800). NS3 protein of Bluetongue is a viroporin that is critical for virus release (Han et al., J. Biol. Chem., 2004, 279, 41, 43092-43097). In addition, picronaviruses (Gonzalez et al., FEBS Lett., 2003, 552, 28-34), African horse sickness, and Japanese encephalitis encode proteins with viroporin activity that play central roles in viral pathogenesis (Van Niekerk et al., Virology, 2001, 279, 499-508; Chang et al., J. Virol., 1999, 73(8), 6257-6264).
Influenza viruses infect the upper and lower respiratory tracts and cause substantial morbidity and mortality annually. Influenza A viruses, which also infect a wide number of avian and mammalian species, pose a considerable public health burden with epidemic and pandemic potential. Influenza together with complications of the virus is consistently among the top 10 common causes of death, ranking higher than some other much more widely publicized killers, such as the HIV virus that causes AIDS. It is estimated that in annual influenza epidemics, 5-15% of the world's population contracts influenza, resulting in an estimated 3-5 million cases of severe illness and 250,000 to 500,000 deaths around the world from influenza-associated complications. In the U.S., 10%-20% of the population is infected with the flu every year, with an average 0.1% mortality. The flu causes 36,000 deaths each year in the U.S., and 114,000 hospitalizations. The cost of influenza epidemics to the U.S. economy is estimated at $3-15 billion. Approximately 20% to 40% of the world's population became ill during the catastrophic “Spanish” flu pandemic in 1918, which killed an estimated 40 to 50 million people worldwide and 675,000 people in the United States. The “Asian” flu pandemic of 1957 resulted in the deaths of approximately 69,800 people in the United States and 2.0 to 7.4 million worldwide. The H1N1 swine flu pandemic in 2009 has caused about 3,000 deaths worldwide to date.
Tamiflu (oseltamivir), which targets neuraminidase protein, is the only remaining orally administered anti-flu drug on the market and resistance to the drug is increasing with oseltamivir-resistant viruses arising during clinical use of the drug in children (Kiso et al., Lancet, 2004, 364, 759-65). Oseltamivir has been used for treatment of infected individuals and although it is FDA-approved for prophylaxis its usefulness for prophylactic treatment has been questioned in a recent systematic analysis of data from 51 controlled trials (Jefferson et al., Lancet, 2006, 367, 303-13). Thus, there is an immediate need to develop additional agents that inhibit the M2 proton channel and its drug-resistant forms, and in particular the most prevalent mutant form, S31N, but also in others including L26, V27, A30, and G34.
Influenza A and B viruses each encode a small oligomeric integral membrane protein, M2 of influenza A virus and BM2 of influenza B virus, each of which is a proton-selective ion channel. The M2 protein plays an important role during the early and late stages of the viral life cycle. Early in the cycle, the virus enters cells by receptor-mediated endocytosis, which places the virus into endosomal vesicles. Proton-pumping ATP-ases in the endosomal membrane lower the internal pH, which triggers the fusion of the viral envelope with the endosomal membrane and the release of the viral RNA into the cytoplasm. However, unless the inside of the virus is acidified prior to fusion, the RNA remains encapsulated by a matrix protein known as M1 (Ito et al., J. Virol., 1981, 65, 5491-8). The M2 protein provides a conduit for passage of protons into the interior of the virus, thereby promoting the dissociation of RNA from its matrix protein. This is a crucial step in uncoating of the virus and exposing its content to the cytoplasm of the host cell. In some strains of influenza A virus, the M2 protein is also important for equilibrating the pH of the lumen of the Golgi apparatus with the cytoplasm, thus preventing a premature conformational change in the viral hemagglutinin at the wrong time and in the wrong place (Ciampor et al., Acta Virologica, 1995, 39, 171-181). Inhibition of M2 at this later stage of the viral life cycle prevents viral maturation and release from the host cell.
Several features make M2 an excellent target for an anti-influenza drug. It is essential and present in all known isolates of influenza A virus, and it is already validated as a drug target. Although a variety of mutations occur naturally and can be isolated in cell culture, one mutant in particular, S31N, predominates in more than 98% of the transmissible resistant viral strains isolated from patients in the last decade (Bright et al., Lancet, 2005, 366, 1175-1181).
Thus, there is a great need for additional compositions and methods of treatment based on the use of antiviral compounds against key viral pathogens and, optionally, less prone to the development of resistance by those pathogens. Moreover, there is a great need for additional compositions and methods of treatment based on the use of antiviral compounds that are effective in the treatment of viral pathogens that have already developed resistance to existing antiviral agents. In particular, there is a great need for effective compositions and methods for the treatment of viral infections such as influenza, Ebola, Marburg, bluetongue, foot and mouth disease, African horse sickness, and Japanese encephalitis (including the strains that have already developed resistance to existing antiviral agents). The present invention is directed to these and other important ends.